Electron schreef op 03-Mar-15 om 10:15 AM:
> Hi Wouter,
>
> At 16.27 2015.02.28, Wouter van Ooijen wrote:
>> Electron schreef op 28-Feb-15 om 11:10 AM:
>>> Hello,
>>> please take a look at the following circuit:
>>> http://oi57.tinypic.com/256h0fa.jpg
>>>
>>> I use a high voltage (500V) PNP transistor as a high side switch.
>>> With a PIC pin I turn it ON or OFF, through the NPN high voltage
>> (500V) transistor.
>>> The 200K resistor is to partially turn ON the transistor when the
>> PIC is in tri-state (e.g. during brief reset time). Then the PIC will
>> be either ON or OFF, so it will be able to withstand also the leakage
> >from that 200K resistor (2 mA worst case, the protection diodes can
>> withstand it). 200K and 2K resistors are of course rated for 400V
>> (actually I use a couple of 200V equal resistors (half resistance) in se=
rie).
>>> Basicly the PIC knows the voltage of both sides, and turns ON the
>> switch when the left side voltage is higher than the right side
>> voltage, with some exceptions.
>>> It's like a diode, but being in software it allows me to do things
>> that a diode cannot do.
>>> Switching speed can be in the KHz region. The impedance is not
>> really low (currents are 100 mA peak max, and limited by the source anyw=
ay).
>>> Now the question: do you think that the Rb resistor is really necessary=
?
>>> If so, why?
>>>
>> Static case: Without that resistor the off-current (leakage) of the NPN
>> will be amplified by the worst case (in this case: highest) Hfe of the
>> PNP. The resulting current will flow through the PNP, even when it is
>> supposed to be off. The Rb is there to 'take' this current without
>> causing the BE junction of the PNP to conduct. So this resistor must be
>> less than 0.6V/(NPN leakage).
>>
>> Dynamic case: When the NPN switches off, the charge on the base of the
>> PNP will keep it conduction for some time. The Rb resistor will amek it
>> switch off faster.
>>
>> Suggestion: add a 5V zener from NPN base to ground, and put the 2k in
>> the emitter lead. That makes the NPN stage a constant current sink, so
>> you will have more predictable properties.

If 3.3V, make the zender 3.3V too.

Choose the emitter resistor as 2.7V / (PNP base current).
> But will it still be possible to turn it ON and OFF via the (3.3V) PIC?
>



Yes, of course.

Wouter

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